Simulation Of An Acoustically Induced Electromagnetic Field In A Borehole Embedded In A Porous Formation

Abstract

When an acoustic point source located on the borehole axis emits an acoustic wave, an electric field, as well as an acoustic field, is generated in the porous formation around the borehole due to an electrokinetic effect. The coupled acoustic and electromagnetic wavefields were formulated by applying Pride's (1994) governing equations and boundary conditions at the borehole wall. Numerical examples show that two kinds of electric fields can be received on the borehole axis. The propagating electromagnetic wave arrives at different receivers almost simultaneously, and appears as the first wave packet in the full electric field waveform. Another kind of electric field accompanies the acoustic pressures, and consists of the same component waves as that of the acoustic waveforms. On the borehole axis, the coupled electric field vector is in the axial direction on the borehole axis. A study of the ratio of the magnitude of electric field strength to the magnitude of pressure, or REP, shows that the compressional wave has the largest REP value, the Stoneley wave the next, and the shear wave the smallest. The peak value of the electric field strength at 1 kHz is about 100 times larger than that at 10 kHz, while the REP at 1 kHz is about five times larger than REP at 10 kHz. Off the borehole axis, the electric field has a radial and axial component, and one Can also receive a circumferential magnetic field. When the interdependence between porosity, tortuosity and permeability is ignored, the REP increases rapidly with porosity, decreases with tortuosity, and changes little with permeability. The electric field strength decreases with borehole fluid salinity and formation water salinity.